1. Field of the Invention
The present invention relates to a fixing apparatus for fixing a toner image onto a recording medium, an image forming apparatus including the fixing apparatus, and a method for controlling the image forming apparatus, and more particularly, a fixing apparatus, an image forming apparatus (e.g. a copier, a digital multi-function machine, a printer), and a method for controlling the image forming apparatus for fixing a toner image onto a recording medium such as roll paper, long paper having a length greater than standard size paper, wide paper having a width greater than standard size paper.
2. Description of the Related Art
Image forming apparatuses such as copiers and printers form images on recording media such as plain paper and OHP (Over Head Projector) sheets. In forming the images, an electrophotographic method is widely used for attaining satisfactory image forming speed, image quality, and cost. In using the electrophotographic method, a toner image is formed on a recording medium, and is then fixed onto the recording medium by applying thermal pressure thereto. A heat roll method is currently the most commonly used fixing method from the aspects of, for example, safety. In using the heat roll method, a cooperative pressing part (also referred to as a nipping part) including a fixing roller heated by a heating member (e.g. a halogen heater) and a pressing roller facing the heating roller is formed so that the toner image transferred onto the recording medium can be heated by the nipping part.
In recent years, image forming apparatuses, which can form images on long length paper such as roll paper or long paper having a length longer than standard size paper, have been introduced. In a case of forming images on long length paper, a fixing roller having a large heat capacity is required owing to the fact that the heat required for fixing an image onto the recording medium is continuously absorbed for a long period. However, the use of a fixing roller having a large heat capacity increases the time required for initiating the process of conveying the recording medium through the nipping part (rise time). Conventionally, in order to prevent this problem, an auxiliary heating part is disposed inside the fixing roller and a comparatively large amount of power is supplied to the auxiliary heating part at the early stages of the rise by using a rechargeable auxiliary power supply, so that the fixing roller can be heated without requiring a large heat capacity and the time required for initiating the process of conveying the recording medium through the nipping part (rise time) can be shortened.
For example, Japanese Laid-Open Patent Application No. 2002-14574 discloses an image forming apparatus having plural heating parts provided to a fixing roller for driving the heating parts during a period of rising temperature from a standby state and generating a large amount of power.
In another example, Japanese Laid-Open Patent Application No. 2004-286869 discloses an image forming apparatus having a large capacity capacitor serving as the auxiliary power supply of a fixing roller and efficiently charging the capacitor by detecting the charge voltage of the capacitor.
In another example, Japanese Laid-Open Patent Application No. 2005-32558 discloses an image forming apparatus that controls the power supplied to an auxiliary heating part based on the charge voltage of a capacitor and the temperature of a fixing roller for providing a sufficient amount of power to the auxiliary heating part while preventing the supplied power from adversely affecting the circuits surrounding the auxiliary heating part.
In another example, Japanese Laid-Open Patent Application No. 2005-18049 discloses an image forming apparatus that controls a charging operation of a capacitor and an image forming job for minimizing the time for completing the image forming job (including the time for executing a copying job and the time for charging the capacitor).
However, even if the rise time for initiating the process of conveying the recording medium through the nipping part is shortened by using the above-described auxiliary power supply, an image cannot be sufficiently fixed onto a long recording medium as long as a fixing roller (fixing member) having a small thermal capacity is used owing to the fact that the temperature of the fixing roller drops below the temperature required by the fixing roller to fix the image onto the recording medium. This results in poor fixing quality.
Furthermore, in a case where a fixing member having a large thermal capacity is used, a long period of time is required for heating the fixing member to a temperature sufficient for fixing a toner image onto a recording medium. As a result, a long period of time is required for starting a fixing operation.
Japanese Laid-Open Patent Application No. 6-4005 discloses an image forming apparatus having a determining part for determining whether the conveyed recording medium is long paper that is greater in length than standard size paper. According to the determination results of the determining part, the number of heaters that are lit on/off is controlled.
However, since this method determines whether the heaters are to be turned on and off based on merely the size of the recording medium, too much heat may be supplied to the fixing roller where the fixing roller is already heated to a sufficient temperature. As a result, a stable fixing performance cannot be attained.
Japanese Laid-Open Patent Application No. 8-220928 discloses an image forming apparatus that prevents deterioration of image quality due to the drop of fixing temperature when passing a long recording medium through a nipping part by changing the fixing conditions according to paper size, paper thickness, and surface characteristics.
This method determines the heating conditions of the fixing roller based on merely the characteristics of the target recording medium. However, fixing quality also depends on, for example, heating conditions of the fixing roller that differ based on the characteristics of toner or its environment. Therefore, it is difficult to constantly attain satisfactory results by using this method.
Other related examples showing a configuration of an image forming apparatus including a fixing apparatus are disclosed in Japanese Laid-Open Patent Application Nos. 2001-092295, 2001-154525, 2002-184554, and 2005-39873.